1. Field of the Invention
The present invention relates to a surface illuminant equipment used in a back light for a liquid crystal display or the like.
2. Description of the Related Art
As this type of surface illuminant equipment, there has been commonly known one in which a side surface of a light guiding plate formed from a translucent material, such as an acryl resin or the like, is set as a light incidence plane for a light from a light source, the light from the light source disposed opposite to the light incidence plane is guided to the light guiding plate and a part of the source light is emitted from a front surface of the light guiding plate serving as a light emitting plane (for example, refer to Japanese Patent Laid-open No. 2000-221329 and No. H5-203947).
In the surface illuminant equipment, to the front surface (light emitting plane) side of the light guiding plate, there is provided an optical sheet, such as a prism sheet or a dispersion sheet, for adjusting the propagation course of the light emitted from the front surface; to a back surface side of the light guiding plate, there is provided a reflection sheet for reflecting the light emitted from the back surface toward the front surface. One of the front surface and the back surface of the light guiding plate is formed with a dot pattern (a pattern dotted with micro concaves or micro convexes) for reflecting the source light guided to the inner side of the light guiding plate and transmitting therein by printing, machining or the like. In order to uniformize the luminance of the light emitted from the front surface of the light guiding plate, the dot pattern is so disposed that the density of the dot pattern (numbers of dots per unit area) and the size of each dot vary with respect to a distance to the light source.
The other one of the front surface and the back surface of the light guiding plate (surface opposite to the surface formed with the dot pattern), as disclosed in Japanese Patent Laid-open No. 2000-221329 and No. H5-203947, may be formed with a prism extending along the guiding direction (a direction vertical to the incidence plane) of the source light to the light guiding plate. However, the other surface may also be formed to have a mirror shape.
However, in the conventional surface illuminant equipment, each dot in the dot pattern is a convex or concave formed with respect to the flat front surface or back surface of the light guiding plate. Therefore, for the surface illuminant equipment having the dot pattern formed on the flat front surface or back surface of the light guiding plate, there are such problems as described in the following.
That is to say, after the source light has been guided to the light guiding plate, when the front surface (light emitting plane) of the light guiding surface is viewed from the width direction thereof, those dotted portions with relatively high luminance are visibly observed sparkling on the front surface of the light guiding plate at the respective dot position. In other words, there arises luminance irregularity like the dot pattern for the luminance distribution on the front surface of the light guiding plate. Moreover, when the front surface of the light guiding plate is viewed via the prism sheet disposed on the front surface side, the dotted portions are dispersed by the prism sheet in the direction parallel to prisms of the prism sheet, accordingly, for example as illustrated in FIG. 6, linear portions A with relatively low luminance are visibly observed locally.
Consequently, when the conventional surface illuminant equipment is used as, for example, a back light for a liquid crystal display, there is a problem that only parts of an image corresponding to the linear portions are visibly observed and a display irregularity happens to the image displayed on the liquid crystal display.
The reason why the light having the dotted portions with relatively high luminance is emitted from the front surface of the light guiding plate as described above may be considered as the follows. An edge for each dot has roundness to some extent, thus while the edge is functioning as a condensing lens, an incident light to the edge is not reflected to the desired direction but scattered and it is easier for the light to be emitted to the width direction of the light guiding plate or a direction close to the width direction. Thereby, the light with relatively high luminance at each dot position is emitted to the width direction of the light guiding plate or a direction close to the width direction. And the light of the dotted portions emitted from the front surface of the light guiding plate via the edge thereof becomes the linear portions with relatively low luminance since the propagation course thereof has been altered greatly to an inclined direction with respect to the width direction of the light guiding plate in the prism sheet.
Moreover, the conventional surface illuminant equipment has the following problem. In the case where the surface illuminant equipment is used, for example, as a back light for a liquid crystal display, the back surface of the light guiding plate is provided with various units such as a driver board for the liquid crystal display, a connector, a signal line and others with a reflection sheet disposed therebetween. Therefore, those units interfere (collide) with the back surface of the light guiding plate via the reflection sheet by vibration, impact and others, exerting an acting force on the light guiding plate or the like of the surface illuminant equipment between the liquid crystal display on the front surface and the units on the back surface of the light guiding plate.
Thus, in the case where the dots of the dot pattern on the light guiding plate are micro convex portions, especially in a location where the density of the dots is small, due to the interference between the light guiding plate and the units, it is easy for the acting force to be concentrated on each dot. As a result, there is a problem that the dots are easy to get damaged.